Digital communications having the highest speed or bandwidth employ laser beams as optical carriers generally having carrier frequencies in the terahertz range that permit extremely high modulation bandwidths. The optical carrier signal is modulated by an RF information signal (such as a computer data stream or a video signal or the like) or many information signals on respective sub-carriers. One common method of modulation uses an optical modulator consisting of complementary electrically responsive optical phase shifters in each of two branches of an interferometer such as a Mach-Zehnder interferometer. The optical modulator (the interferometer with the complementary optical phase shifters) is interposed between the optical carrier source (a laser) and the communication channel (a fiber optic cable, for example).
Each electrically responsive optical phase shifter typically consists of an electro-optical element that imposes a phase shift on the optical carrier signal proportional to an applied voltage. The applied voltage is (or is derived from) the information signal. The amplitude range (amplitude deviation) of the applied voltage must be sufficient for the electrically responsive optical phase shifters to impose quarter wavelength phase shifts in opposite directions in each of the two branches of the interferometer (for a total net maximum phase shift between the two branches of a half wavelength) for maximum modulation of the optical carrier. The voltage deviation required to achieve this maximum deviation is referred to herein as the quarter wavelength excursion voltage. An amplifier may be employed to raise the peak modulation voltage to the required amplitude. Typically, quarter wavelength excursion voltage (i.e., the applied voltage peak deviation required to realize a quarter wavelength phase shift range in each optical phase shifter) is on the order of about a volt or more.
Bandwidth requirements for the information signal are constantly increasing, and the modulation frequency must be increased to meet such demands. Unfortunately, it is difficult to amplify the modulation voltage to the required level at extremely high modulation frequencies (e.g., 40 GHz or higher). Thus, information signal bandwidth is limited by the modulation voltage deviation required by each optical phase shifter to attain the desired quarter wavelength phase shift. One way around this problem would be to somehow decrease the voltage swing required by the optical phase shifter to impose the quarter wavelength phase shift. For example, in some types of electrically responsive optical phase shifters, an increase in length of the phase shifter reduces the voltage deviation required to realize a quarter wavelength phase shift on the optical carrier. Unfortunately, such an increase in size increases the effective capacitance of a semiconductor optical phase shifter, so that it is difficult to drive it at the desired high modulation frequency. Thus, it has seemed that the relatively large voltage deviation requirements of a typical optical modulator imposes a limit on the information signal bandwidth.
The invention is embodied in an optical modulator requiring extremely low modulation voltages for full modulation of the optical carrier. The optical modulator consists of an interferometer formed on a substrate as semiconductor optical waveguides with a pair of branches and complementary electrically responsive optical phase shifters in the respective branches and driven by an RF modulating source in a xe2x80x9cpush-pullxe2x80x9d arrangement in one embodiment. The optical phase shifters are themselves semiconductor structures integrated within the waveguides on the substrate. Each optical phase shifter employs the quantum-confined Stark effect to produce quarter wavelength optical phase shifts with extremely low modulation voltages (i.e., lower by one or two orders of magnitude relative to conventional devices). Each optical phase shifter consists of a reverse-biased p-i-n layered structure of electro-optical crystalline material with an array of quantum dots in the intrinsic layer of the PIN structure that promote formation of optically generated excitons in the intrinsic layer. Superimposing the information signal voltage in push-pull fashion on the D.C. bias voltage of the optical phase shifter in each branch causes the exciton energy level spacing therein to shift accordingly (in opposite directions in the two phase shifters) and produces in the complementary optical phase shifters concomitant opposing changes in refractive index (and therefore in phase shift) that follow the information signal amplitude. The deviation in bias voltage required to realize quarter wavelength phase shift in each optical phase shifter is an order of magnitude smaller than conventional optical modulators, leading to an order of magnitude increase in information signal bandwidth beyond current technology, a revolutionary advance in the art.
The D.C. offset of the bias voltage is chosen to coincide with regions of maximum change in refractive index. This choice minimizes the required swing in bias voltage required to realize a quarter wavelength phase shift by each optical phase shifter (e.g., as low as a 45 mV swing). The additional advantage of a large electro-refractive effect is that the electro-optical crystal length required to realize the quarter wavelength phase shift is reduced for a given modulation voltage. This reduced length reduces insertion losses. The RF frequency of the modulation signal is ultimately limited by the capacitive reactance of the modulator, which is proportional to the length of the modulator, as well as other parameters. As the modulation frequency increases, the modulator capacitive reactance decreases. When the modulator capacitive reactance equals the load resistance, the modulator efficiency falls to 50%. Hence the cut-off frequency is defined as that frequency for which the modulator capacitive reactance equals the load resistance (50 Ohms). The capacitance can be adjusted by adjusting various parameters of each optical phase shifter, such as the thickness of the intrinsic layer of the electro-optical PIN structure.
In one embodiment, a quantum-confined Stark effect optical modulator includes an interferometer having a beam splitter, first and second parallel optical branches fed by the beam splitter and a beam combiner fed by the first and second parallel optical branches and a laser for feeding a laser beam to the beam splitter. First and second optical phase shifters are provided in respective ones of the first and second parallel optical branches. Each optical phase shifter includes an intrinsic semiconductor crystalline planar layer and p-type and n-type planar semiconductor layers on opposite faces of the intrinsic semiconductor crystalline planar layer, the intrinsic layer lying in a plane parallel to a direction of propagation of the laser beam in the respective optical branch. The intrinsic layer has plural layers of planar arrays of quantum dots therein. A reverse bias D.C. voltage source is connected across the p-type and n-type layers. A pair of modulation source terminals is connected across the reverse bias D.C. voltage source whereby a modulation signal modulates the reverse bias voltage across the p-type and n-type layers.
The modulation source terminals of the first and second optical phase shifters are connected in opposite polarity (i.e., in push-pull fashion) to a dual output modulation source. The intrinsic layer undergoes a change in refractive index sufficient to impose a quarter wavelength phase shift on the optical signal in response to an RF modulation signal.
The quantum dots in the intrinsic layer anchor quantum-confined exciton states define an exciton (electron-hole pair) optical absorption line spectrum. The quantum-confined states decrease in energy level upon increase of the reverse bias voltage.
Each of the optical phase shifters produces a quarter wavelength phase shift whenever the reverse bias voltage is shifted in push-pull fashion by the quarter wavelength excursion voltage. The first and second optical phase shifters are operated in push-pull mode whereby a quarter wavelength shift in each optical phase shifter produces a total net phase shift of a half wavelength between the first and second optical phase shifters.
The p-type and n-type layers confine a major portion of the laser beam to the intrinsic layer. The p-type, n-type and intrinsic layers are formed of a semiconductor material having a band gap such that the material is nominally transmitting (low loss) at the wavelength of the laser beam. The quantum dots have a diameter, height and layer-to-layer spacing sufficient to form a potential wells capable of capturing exciton electron-hole pairs with sufficient binding energy to create atomic-like transition energy spectra. The transition energy spectra undergo a Stark effect downward shift in response to an increase in the reverse bias voltage with a corresponding shift in refractive index of the intrinsic layer, the shift in refractive index being sufficient to impose a phase shift on the laser beam in the intrinsic layer equal to a quarter wavelength of the laser beam whenever the increase in the reverse bias voltage reaches a quarter wavelength excursion voltage of the intrinsic layer.